In less than a decade we have learned that bacteria and perhaps prokaryotes in general, possess linear chromosomes and plasmids, as well as multiple, but different chromosomes. Heretofore it had been accepted that prokaryotes contained but a unique, circular chromosome. To date, some 5-7 different genera of bacteria have been found to contain two or more chromosomes and it is interesting that most of these genera have been shown to interact with the cells of higher organisms. This research, as described here, is designed to provide the first experimental approach to understanding this line of microbial development. One approach is based on the observation that several genes have been shown to be heterozygous between chromosomes I and II in the bacterium, Rhodobacter sphaeroides 2.4.1. Using this well studied bacterium we will determine the extent of heterozygosity existing between chromosomes I and II, whether or not both members of a pair of homologous genes are functional and under what growth conditions; what are the kinds and types of truly essential genes which are mapped to each chromosome and finally; is this line of development shared by all members of this group of bacteria and if not, why not? Heterozygosity is known to increase the "fitness" of those populations of higher organisms which show ploidy within their life cycle. Can this be true of prokaryotes, and if so, how? Thus, the extent of heterozygosity in R. sphaeroides 2.4.1 and the conditions under which heterozygous genes are expressed should help to address this question. Although heterozygous gene pairs can also be essential genes, no such prerequisite is necessary. Therefore, the kinds and distribution of essential genes which map to chromosomes I and II may help to reveal the origin(s) of the multi-chromosomal state in R. sphaeroides 2.4.1. Finally, because several of the gene pairs found to be heterozygous are very similar to gene families found in higher organisms, we may also learn the origins of the eukaryotic type cell and its inclusions.